An active filter is a device that modifies the amplitude and/or phase characteristics of a signal with respect to frequency, and which includes an amplification device to amplify the signal at relatively low frequencies. An active filter may be electrically positioned between a power source and a load, and can help to alleviate power quality issues introduced by harmonic currents and low power factor.
Currently, active filter solutions for industrial applications are available at low rated voltages (i.e., less than or equal to 690 volts). However, existing solutions for active filters at voltage levels above 1000 volts have distinct disadvantages. For example, attempts to provide a hybrid active filter that includes an inverter that is rated for a small fraction of the utility voltage have required large capacitors and expensive magnetic components, and such systems absorb a fixed level of leading reactive power (VARs), which results in poor power factor at medium and light loads.
The use of cascaded or series-connected inverters for compensation of fundamental reactive power (or VARs) is known. However, the circuits proposed to date have limited utility. Other attempts at using series-connected inverters have suggested square-wave mode of operation to reduce the losses in the inverters. However, in such systems, the number of harmonics that can be compensated is limited by the number of series connected inverters, as the higher harmonics require a larger number of inverters.
Accordingly, it is desirable to provide an improved filter for medium-voltage applications.